This invention generally relates to control systems and methods for optical amplifiers, and is specifically concerned with a controller for a multi-channel optical amplifier that achieves and automatically maintains a selected gain level by controlling the average inversion level of the dopant atoms in the gain fiber.
Optical amplifier controllers are known in the prior art. The purpose of such controllers is to achieve and maintain a selected gain level. Such a controller may operate, for example, on the basis of a predetermined lookup table where an empirical determination is made between electrical power conducted to the optical pump and the resulting total amplifier gain. A selected signal gain set point is both achieved and maintained by the digital processor of the controller which uses the lookup table to select the amount of power conducted to the optical pump that correlates with a selected gain set point.
Other optical amplifier controllers are known which utilize photodiodes to continuously monitor the actual input and output power of the amplifier. The digital processor of the controller controls power conducted to the optical pump to modulate pump output power until the signal gain is equal to the selected gain set point. Such a design is advantageously more accurate and reliable than controllers which rely solely upon a predetermined lookup table since a lookup table may not accurately correlate actual signal gain to pump input power due to fluctuations in noise levels, diminishing pump efficiency over time, and different operating conditions of the amplifier.
Unfortunately, all such controllers which achieve and maintain a selected signal gain level by either directly or indirectly monitoring the total output of the amplifier can generate large transient power spikes in the surviving channels of the amplifier output when channels are added or dropped. This is a particularly bad problem in optical networks with dense wavelength division multiplex signals (DWDM), in which a rapid addition or dropping of a large proportion of input channels occurs. Such unwanted spikes come about from the fact that gain figure used by the controller is a total gain figure, from which an average gain for all of the amplifier channels is inferred. However, because the output of most amplifiers is not completely flat across its transmission spectrum, the gain level of some of the channels may be significantly higher than the gain level of other channels. Hence, the adding or dropping of a large proportion of the total number of available channels can result in a substantial, short-term over or under amplification of the surviving channels, thereby creating a transient spike in the amplifier output. Such spikes generate undesirable noise in the network, and can result in the temporary loss of a channel.
To minimize the noise generated by such spikes in DWDM networks, the optical amplifiers have one or more gain equalization filters (GEFs) in order to flatten the gain throughout a channel range that typically encompasses 43 channels equally spaced within a spectrum range of 1529.55 nm to 1563.05 nm. Unfortunately, such GEFs are relatively complex and expensive components. A less expensive alternative would be the use of a number of smaller amplifiers, each having a channel capacity of perhaps eight channels. The smaller channel capacity generates an inherently flatter output, thus obviating the need for GEFs. As a full signal load utilizing all 43 channels seldom occurs in many networks, the use of a fewer number of eight channel amplifiers is a practical alternative whose capacity could be easily expanded as needed by adding more such amplifiers on a “pay as you grow” basis. However, as the output of such optical amplifiers is not completely flat, and as the lower channel capacity makes it even more likely that a large percentage of the channels will be added or dropped during the operation of the network, the problem of noise generation from transient spikes would be even greater.
Clearly, what is needed is an optical amplifier controller which is capable of achieving and maintaining a gain set point which does not generate under-amplification or over-amplification spikes when a large proportion of available channels are added or dropped from the signal. Ideally, such a controller should be simply and inexpensively constructed from preexisting components of control circuitry. It would also be desirable if such a controller could also easily and cheaply provide an indication of the gain for each of the transmitted channels so that gain flattening measures may be taken when necessary.